It is known that liquid crystals have an order of unique molecular orientation depending on their type. Accordingly, liquid crystals are utilized in various fields by utilizing and controlling said molecular orientation, and forms an industrially large field.
With respect to monomeric liquid crystals, those of the nematic type are used widely as display elements in watches, calculators or TV, as is well-known. In the field of display, they have established a firm position. As described above, the majority of the application of monomer liquid crystals is based on electric optical effects.
With respect to polymeric liquid crystals, not only their physical characteristics but also their electric optical effects or thermal optical effects similar to those of monomeric liquid crystals are well-known to be exhibited depending on the respective nematic, smectic and cholesteric liquid crystal types. The response of polymeric liquid crystals to external force such as electric field, heat etc. is slower than that of monomeric liquid crystals, so they cannot be used for the same uses as monomeric liquid crystals. However, polymeric liquid crystals have the major characteristics that orientation structures unique to the respective liquid crystal types can be fixed. Polymeric liquid crystals having an orientation structure unique to the liquid crystals fixed depending on the type of the liquid crystals are used as indicator materials, recording materials etc. The polymeric liquid crystals can be formed into films or thin films advantageous as optical materials, and the polymeric liquid crystals formed into films or thin films can be applied in various fields.
The physical values characteristic of liquid crystals regardless of whether they are monomeric or polymeric include double refractive index (.DELTA.n). The .DELTA.n values of monomeric nematic liquid crystals depends on molecular structures, and tran type (&lt;0.28), azo, azoxy type (0.25 to 0.30), Schiff base type (&lt;0.24) and pyrimidine type (&lt;0.23) show relatively high .DELTA.n values. Cyclohexylcyclohexane type (&lt;0.06), cyclohexylcarboxylic acid ester type (&lt;0.1), and phenylcyclohexane type (0.08 to 0.19) show low .DELTA.n values. The .DELTA.n value is one of important parameters in the field of electronic display, and development of high-performance is made feasible by the characteristics of said physical value. For example, the .DELTA.n of monomeric nematic liquid crystals is raised to reduce the thickness of cells in display devices and to improve response rates. In dual-STN, a raise in the .DELTA.n of liquid crystals is desired to improve display contrast. Although the .DELTA.n value is an important parameter as described above in the field of electronic display, the .DELTA.n values of nematic liquid crystals are usually in the range of about 0.1 to 0.3 so that their further application to electronic displays are limited.
To increase the .DELTA.n value, it is necessary to raise said value by specific molecular structures. It is known that the .DELTA.n value depends on the molecular polarizability and the parameter of orientation order and can thus be improved by compounds with high polarizability generating anisotropy optically or by conjugate compounds with high electron density, such as benzene rings, polycyclic aromatic groups, ethylene-acetylene chain groups, terminal cyano groups etc. On the basis of this, M. Hird et al. have reported monomeric liquid crystals with .DELTA.n of less than 0.43 consisting of polycyclic aromatic tran type liquid crystals with high polarizablility (M. Hird et al., Liquid Crystals, 15, 123 (1993)).
However, polymeric liquid crystals with .DELTA.n values exceeding 0.3 are usually not present and there is only one report where .DELTA.n was raised to 0.865 by mechanically drawing aromatic polyamides (H. G. Rogers et al., Macromolecules, 18, 1058 (1985)).
For application of polymeric liquid crystals to recording mediums, memory elements etc. in the field of optoelectronics, it is evidently necessary to develop polymeric liquid crystals showing both self-orientation over glass transition temperature (Tg) and self-memorization below Tg, but there is no report on such liquid crystal polymers either.
As described above, development of polymeric liquid crystals having high .DELTA.n values due to the self-orientation of the liquid crystals themselves is essential for development for new use and application of polymeric liquid crystals. Further essential requirements for said polymeric liquid crystals are that they are excellent in the orientation ability and fixation ability and are easily formed into films and thin films, and there has been demand for development of such polymeric liquid crystals.